Abstract

Models indicate a time-varying radiative response of the Earth system to CO2 forcing. This variation implies a significant uncertainty in the estimates of climate sensitivity to increasing atmospheric CO2 concentration. In energy-balance models, the temporal variation is represented as an additional feedback mechanism, which also depends on the ocean temperature change. Models and observations also indicate that a spatio-temporal pattern in surface warming controls this additional contribution to the radiative response. Some authors picture the effect as a purely atmosphere-based feedback change, reducing the role of the ocean's enthalpy-uptake variations. For the first time, I derive, using a widely-known linearised conceptual energy-balance model, an explicit mathematical expression of the radiative response and its temporal evolution. This expression connects the spatio-temporal warming pattern to an effective thermal capacity, stemming from changes in the ocean enthalpy uptake. In comparison with more realistic energy-balance frameworks, and unlike the notion of additional feedback mechanisms, I show that an expanded effective thermal capacity better explains the variation of the radiative response, naturally connects with the spatio-temporal surface warming pattern, and provides a non-circular framework to explain the variation of the climate feedback parameter.